Sheet conveyor and image forming system

ABSTRACT

A sheet conveyor includes: a holder that detachably holds a sheet roll; a conveyance roller that conveys, by rotation, a sheet pulled out from the sheet roll; a first motor that rotates the conveyance roller; a second motor that rotates the sheet roll along with the conveyance of the sheet; a tension estimator that estimates tension of the sheet conveyed by the rotation of the conveyance roller; a measuring device that measures a physical quantity related to rotary motion of the conveyance roller; and a controller. The controller estimates acceleration torque of the second motor required for rotating the sheet roll depending on acceleration of the conveyance roller, based on rotation acceleration of the conveyance roller specified by the physical quantity measured by the measuring device, and calculates a feedforward control input for the second motor based on the acceleration torque estimated.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priorities from Japanese Patent Application No. 2019-226415 filed on Dec. 16, 2019 and Japanese Patent Application No. 2020-039179 filed on Mar. 6, 2020, the disclosures of which are incorporated herein by reference in their entireties.

BACKGROUND Field of the Invention

The present disclosure relates to a sheet conveyor and an image forming system.

Description of the Related Art

There is conventionally known a recording apparatus that records an image on a sheet pulled out from a sheet roll (see, for example, Japanese Patent Application Laid-open No. 2013-091219). The sheet is pulled out toward a recording head by a conveyance roller.

In this recording apparatus, a diameter of the sheet roll is measured by an optical sensor to synchronize a sheet velocity in an outer circumference of the sheet roll with a sheet velocity in the conveyance roller. A target value of a rotation velocity of the sheet roll is calculated based on the diameter of the sheet roll measured. Further, a control input for a motor that rotates the sheet roll is corrected based on rotation acceleration of the conveyance roller in order to rotate the sheet roll according to the acceleration of the conveyance roller.

SUMMARY

In a sheet conveyor that conveys a sheet pulled out from a sheet roll, weight of the sheet roll changes depending on consumption of the sheet. The change in the weight may change a control input for a motor that is suitable for controlling acceleration motion of the sheet roll to target motion.

In conventional techniques, the motor is controlled without reflecting the change in the weight of the sheet roll. It has been thus difficult to appropriately perform the conveyance control of the sheet under a condition that the sheet is conveyed with an acceleration process.

An object of the present disclosure is to provide a sheet conveyor that is capable of appropriately executing conveyance control of a sheet with an acceleration process under an environment where weight of a sheet roll changes depending on consumption of the sheet.

According to a first aspect of the present disclosure, there is provided a sheet conveyor, including: a holder configured to detachably hold a sheet roll; a conveyance roller configured to convey, by rotation, a sheet pulled out from the sheet roll; a first motor configured to rotate the conveyance roller; a second motor configured to rotate the sheet roll along with conveyance of the sheet; a tension estimator configured to estimate tension of the sheet conveyed by the rotation of the conveyance roller; a measuring device configured to measure a physical quantity related to rotary motion of the conveyance roller; and a controller, wherein the controller is configured to: estimate acceleration torque of the second motor required for rotating the sheet roll depending on acceleration of the conveyance roller, based on rotation acceleration of the conveyance roller specified by the physical quantity measured by the measuring device; calculate a feedforward control input for the second motor based on the acceleration torque estimated; calculate a feedback control input for the second motor based on target tension and estimated tension as the tension estimated by the tension estimator; control the second motor based on at least one of the feedback control input calculated and the feedforward control input calculated; and control the second motor at least based on the feedforward control input from among the feedback control input and the feedforward control input under a condition that the conveyance roller is accelerated.

According to a second aspect of the present disclosure, there is provided a sheet conveyor, including: a holder configured to detachably hold a sheet roll; a conveyance roller configured to convey, by rotation, a sheet pulled out from the sheet roll; a first motor configured to rotate the conveyance roller; a second motor configured to rotate the sheet roll along with the conveyance of the sheet; a tension estimator configured to estimate tension of the sheet conveyed by the rotation of the conveyance roller; a measuring device configured to measure a physical quantity related to rotary motion of the conveyance roller; and a controller, wherein the controller is configured to: control the first motor, based on the physical quantity measured by the measuring device, such that the conveyance roller rotates in accordance with a velocity profile; estimate acceleration torque of the second motor required for rotating the sheet roll depending on acceleration of the conveyance roller, based on target rotation acceleration of the conveyance roller specified by the velocity profile; calculate a feedforward control input for the second motor based on the acceleration torque estimated; calculate a feedback control input for the second motor based on target tension and estimated tension as the tension estimated by the tension estimator; control the second motor based on at least one of the feedback control input calculated and the feedforward control input calculated; and control the second motor at least based on the feedforward control input from among the feedback control input and the feedforward control input, under a condition that the conveyance roller is accelerated.

According to a third aspect of the present disclosure, there is provided an image forming system including: the sheet conveyor as defined in the first aspect or the second aspect and a recording unit. The recording unit can form an image on the sheet conveyed by the sheet conveyor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a configuration of an image forming system according to a first embodiment of the present disclosure.

FIG. 2 illustrates functions of a tensioner.

FIG. 3 is a block diagram of an electrical configuration of the image forming system.

FIG. 4 is a flowchart of a process executed by a main controller.

FIG. 5 is a block diagram of a velocity controller.

FIG. 6 is a block diagram of a configuration of a tension controller according to the first embodiment.

FIG. 7 is a flowchart of a process executed by a gain setter.

FIG. 8 is a block diagram of a configuration of a tension controller according to a second embodiment.

FIG. 9 is a block diagram of a configuration of a feedforward controller according to the second embodiment.

FIG. 10 is a flowchart partially indicating a process executed by the main controller in the second embodiment.

DESCRIPTION OF THE EMBODIMENTS First the Embodiment

A first embodiment of the present disclosure will be described with reference to the accompanying drawings. An image forming system 1 of this embodiment depicted in FIG. 1 is configured so as to form an image on a sheet Q pulled out from a sheet roll Q0 (rolled sheet). The image forming system 1 functions as a sheet conveying device for conveying the sheet Q toward the lower side of the recording head 40.

The image forming system 1 includes a holder 10 (see FIG. 3), a tensioner 15, a conveying roller 20, a nip roller 25, a belt mechanism 30, and a recording head 40. In FIG. 1, configuration downstream of the belt mechanism 30 in a conveyance direction of the seat Q is omitted. Downstream of the belt mechanism 30, for example, a cutter for cutting the sheet Q is provided.

The holder 10 removably holds the sheet roll Q0. The sheet roll Q0 is formed by the sheet Q wound around a hollow core material. The holder 10 includes a rotation shaft 10A inserted through the core material of the sheet roll Q0, and a holder main body (not depicted in the drawing(s)) that rotatably holds the rotation shaft 10A.

The holder main body is fixed in a casing (not depicted in the drawing(s)) of the image forming system 1. The core material of the sheet roll Q0 is fixed so as not to slide with respect to the rotation shaft 10A of the holder 10. As a result, the sheet roll Q0 rotates together with the rotation shaft 10A of the holder 10.

After the sheet roll Q0 is mounted on the rotating shaft 10A, the sheet Q is manually pulled out from the sheet roll Q0 via the tensioner 15 to a position where the sheet Q has passed through the nip point P2 between the conveying roller 20 and the nip roller 25. Point P1 in FIG. 1 indicates the point at which the sheet Q is pulled out of the sheet roll Q0.

The conveying roller 20 conveys the sheet Q nipped between the conveying roller 20 and the nip roller 25 in the conveyance direction indicated by the thick arrow. As depicted in FIG. 2, the tensioner 15 is connected to the spring member 16 to be displaceable in a front-rear direction. The front side corresponds to the downstream side of the conveyance direction and the rear side corresponds to the upstream side of the conveyance direction.

The tensioner 15 is disposed at the rear side of the spring material 16. The tensioner 15 applies tension to the sheet Q by backward urging force from the spring material 16.

The tensioner 15 is displaced frontward by receiving pressing force from the sheet Q under a condition that the sheet Q pulled out from the sheet roll Q0 is conveyed by the conveyance roller 20. The pressing force from the sheet Q corresponds to tension of the sheet Q. FIG. 2 depicts a state where the tensioner 15 and the sheet Q are displaced from a position depicted by solid lines to a position depicted by dotted lines due to the pressing force from the sheet Q.

A position in the front-rear direction of the tensioner 15 is stabilized at a position where the urging force of the spring material 16 and the tension of the sheet Q are balanced. The image forming system 1 is configured to estimate the tension of the sheet Q and control the tension by using the position of the tensioner 15 as an index (details are described below).

The belt mechanism 30 is disposed downstream of the conveyance roller 20 in the conveyance direction. The belt mechanism 30 conveys the sheet Q conveyed from the conveyance roller 20 further downstream. As depicted in FIG. 1, the belt mechanism 30 includes a driving roller 31, a driven roller 32, and a belt 33 stretched between the driving roller 31 the driven roller 32. The driving roller 31 rotates the belt 33 by rotary motion synchronized with the conveyance roller 20.

The belt mechanism 30 further includes the first facing roller 35 and the second facing roller 36. The first facing roller 35 faces the driving roller 31 with the belt 33 interposed therebetween. The second facing roller 36 faces the driven roller 32 with the belt 33 interposed therebetween.

The sheet Q conveyed from the conveyance roller 20 passes between the first facing roller 35 and the belt 33 due to the rotation of the belt 33, so that the sheet Q is conveyed downstream. The sheet Q is conveyed further downstream by passing between the second facing roller 36 and the belt 33. A conveyance velocity of the sheet Q by the rotation of the belt 33 is the same as a conveyance velocity of the sheet Q by use of the conveyance roller 20.

For example, the belt mechanism 30 can have an air absorbing function. That is, the belt 33 may have fine holes through which air passes. A suction device (not depicted) that suctions air may be provided below the belt 33. The sheet Q may be conveyed while being sucked or absorbed to a surface of the belt 33 by the air suction performed by the suction device.

The recording head 40 is provided above the belt mechanism 30 to form an image on the sheet Q passing below the recording head 40. The recording head 40, which is a line head, simultaneously forms images for an entirety in a line direction of the sheet Q passing below the recording head 40. The line direction is a direction along the surface of the sheet Q. The line direction is the conveyance direction of the sheet Q, in other words, a direction orthogonal to a longitudinal direction of the sheet Q.

The recording head 40 may be, for example, an ink-jet head that forms an image on the sheet Q in accordance with an ink-jet system. The recoding head 40 may be a thermal head that forms an image on the sheet Q in accordance with a thermosensitive system or a thermal transfer system.

Under a condition that the recording head 40 forms an image in accordance with the ink-jet system, a fixer 45 for drying and fixing ink may be provided downstream of the recoding head 40 in the conveyance direction. As depicted by the dotted line in FIG. 1, the fixer 45 may be provided above the belt mechanism 30 such that the fixer 45 is adjacent to the recording head 40.

Subsequently, an electrical configuration of the image forming system 1 is explained in detail. As depicted in FIG. 3, the image forming system 1 includes the controller 50 that controls an entire system. The image forming system 1 further includes a supply motor 61, a motor driver 63, a rotary encoder 65, and a measurement circuit 67 to control rotation of the sheet roll Q0 installed in the holder 10.

The supply motor 61 is connected to the rotation shaft 10A of the holder 10 via a gear (not depicted). The supply motor 61 applies power to the rotation shaft 10A. The rotation shaft 10A of the holder 10 rotates under a condition that the rotation shaft 10A receives power from the supply motor 61. The sheet roll Q0 rotates along with the rotation shaft 10A.

The supply motor 61 may be a direct-current motor (DC motor). The supply motor 61 drives the rotation shaft 10A by generating rotation torque depending on a drive current input from the motor driver 63. The motor driver 63 inputs, to the supply motor 61, a drive current depending on a control input U_(SR) input from the controller 50.

The rotary encoder 65 is provided in the rotation shaft 10A of the holder 10 or a rotation shaft of the supply motor 61. The rotary encoder 65 outputs an encoder signal depending on rotation. The measurement circuit 67 measures a rotation position and a rotation velocity (i.e., rotation angle and angular velocity) of the sheet roll Q0 as a physical quantity related to the rotary motion of the sheet roll Q0. The measurement circuit 67 inputs, to the controller 50, the rotation position and the rotation velocity measured. The rotation position and the rotation velocity of the rotation shaft 10A correspond to the rotation position and the rotation velocity of the sheet roll Q0.

The image forming system 1 further includes a conveyance motor 71, a motor driver 73, a rotary encoder 75, and a measurement circuit 77 as the configuration for controlling the rotation of the conveyance roller 20.

The conveyance motor 71 may be a direct-current motor (DC motor). The conveyance motor 71 is connected to the conveyance roller 20 via a gear. The conveyance motor 71 rotates and drives the conveyance roller 20 by generating rotation torque depending on a drive current input from the motor driver 73. The motor driver 73 inputs, to the conveyance motor 71, a drive current depending on a control input UPF input from the controller 50.

The rotary encoder 75 is provided in a rotation shaft of the conveyance roller 20 or a rotation shaft of the conveyance motor 71. The rotary encoder 75 outputs an encoder signal depending on rotation. The measurement circuit 77 measures a rotation position and a rotation velocity (i.e., rotation angle and angular velocity) of the conveyance roller 20 as a physical quantity related to the rotary motion of the conveyance roller 20. The measurement circuit 77 inputs, to the controller 50, the rotation position and the rotation velocity measured.

The image forming system 1 further includes a position detector 80 that detects a position of the tensioner 15. The position detector 80 detects a position X in the front-rear direction of the tensioner 15 with reference to a predefined origin position. The position detector 80 inputs the detected position X to the controller 50. The position detector 80 may be configured, for example, by a linear encoder.

The image forming system 1 further includes a head driver 90, a registration sensor 91, a distance sensor 93, a user interface 95, and a communication interface 97. The head driver 90 is configured to drive the recording head 40 in accordance with a control signal from the controller 50. The registration sensor 91 is provided upstream of the belt mechanism 30. The registration sensor 91 is configured to detect a leading edge of the sheet Q passing therethrough, and to input a detection signal to the controller 50.

The distance sensor 93 is disposed at a position facing the sheet roll Q0. The distance sensor 93 is configured to measure a distance between the distance sensor 93 and the surface of the sheet roll Q0 and to input a measurement signal to the controller 50. For example, the distance sensor 93 is capable of measuring the distance between the distance sensor 93 and the surface of the sheet roll Q0 by irradiating the surface of the sheet roll Q0 with light and receiving its reflection light. The distance sensor 93 may be a sensor that measures a distance by use of ultrasonic waves.

The user interface 95 includes a display section for displaying a variety of information for a user and an input section for receiving instructions from the user. The display section is, for example, a liquid crystal display. The input section is, for example, a touch panel on the liquid crystal display.

The communication interface 97 is configured to communicate with an information device in the wired or wireless communication. The communication interface 97 may be a USB interface or a wired/wireless LAN interface. The information device may be a personal computer or a tablet terminal owned by the user.

The controller 50 includes a main controller 51, a printing controller 53, a velocity controller 55, and a tension controller 57. The main controller 51 includes a processor 51A and a memory 51B. The memory 51B includes a Random Access Memory (RAM) and a flush memory.

The processor 51A executes a variety of processes in accordance with computer programs stored in the memory 51B. In the following, it can be understood that processes executed by the main controller 51 are executed by the processor 51A in accordance with the computer program(s).

The printing controller 53, the velocity controller 55, and the tension controller 57 are configured, for example, by an ASIC. Image data of a printing object is input from the main controller 51 to the printing controller 53. The printing controller 53 inputs, to the head driver 90, a control signal for causing the recording head 40 to print an image based on the image data of the printing object.

The velocity controller 55 determines the control input UPF for the controller motor 71 so that the conveyance roller 20 rotates at a target rotation velocity in accordance with an instruction from the main controller 51. The velocity controller 55 inputs, to the motor driver 73, the control input UPF determined. In this embodiment, the control velocity of the sheet Q is controlled by controlling the rotation velocity of the conveyance roller 20.

The tension controller 57 determines the control input U_(SR) for the supply motor 61 so that the sheet Q is conveyed while having target tension in accordance with an instruction from the main controller 51. The tension controller 57 inputs, to the motor driver 63, the control input U_(SR) determined.

Under a condition that the main controller 51 receives a printing instruction and image data of a printing object from an information device through the communication interface 97, the main controller 51 executes processes indicated in FIG. 4. The main controller 51 controls respective sections of the image forming system 1 in cooperation with the printing controller 53, the velocity controller 55, and the tension controller 57 so that an image based on the image data of the printing object received is formed on the sheet Q.

Under a condition that the processes indicated in FIG. 4 are started, the main controller 51 executes an initial process (S110). The initial process includes a process for arranging the leading edge of the sheet Q at a predefined starting point. The starting point may be a position where the leading edge of the sheet Q is detected by the registration sensor 91, or a position shifted downstream in the conveyance direction by a predefined distance from the position where the leading edge of the sheet Q is detected by the registration sensor 91.

The starting point may be a position where the leading edge of the sheet Q enters the belt mechanism 30. Alternatively, the starting point may be a point that is upstream in the conveyance direction from a position, where image formation is performed on the sheet Q by the recording head 40, by a distance required for acceleration of the sheet Q.

After arranging the leading edge of the sheet Q at the starting point, the main controller 51 starts a conveyance process of the sheet Q (S120). In the conveyance process, the main controller 51 inputs a velocity profile to the velocity controller 55. The velocity controller 55 controls the rotation velocity of the conveyance roller 20 in accordance with the velocity profile. The velocity profile indicates a target rotation velocity of the conveyance roller 20 until the sheet Q is stopped at a target stop position.

Specifically, the velocity profile indicates a target rotation velocity in an acceleration section, a target rotation velocity in a constant velocity section, and a target rotation velocity in a deceleration section. The sheet Q accelerates until the sheet Q reaches a predefined velocity by controlling the rotation velocity of the conveyance roller 20 in accordance with the velocity profile. After reaching the predefined velocity, the sheet Q moves at a constant velocity, and then decelerates.

In the conveyance process, the main controller 51 further inputs a tension profile to the tension controller 57. The tension controller 57 executes tension control of the sheet Q in accordance with the tension profile. The tension profile indicates target tension until the sheet Q is stopped at the target stop position.

After starting the conveyance process, the main controller 51 waits until the sheet Q reaches the predefined velocity (S130). The main controller 51 starts the printing process under a condition that the sheet Q starts constant velocity movement (S140). In the printing process, the main controller 51 causes the printing controller 53 to execute drive control of the recording head 40 for forming the image based on the image data of the printing object on the sheet Q. The recording head 40 repeatedly executes the image forming operation in the line direction in synchronization with movement in the conveyance direction of the sheet Q.

The main controller 51 executes an ending process (S160) under a condition that the printing process and the conveyance process are completed (S150: Yes). The ending process includes a process in which the user is informed of the completion of printing through the user interface 95. Then, the main controller 51 ends the processes indicated in FIG. 4.

Referring to FIG. 5, a detailed configuration of the velocity controller 55 is explained below. The velocity controller 55 calculates the control input UPF for the conveyance motor 71 based on a deviation between the rotation velocity of the conveyance roller 20 measured by the measurement circuit 77 and the target rotation velocity. The velocity controller 55 is configured to execute feedback control for the conveyance roller 20 based on the control input UPF calculated. In the following, the actual rotation velocity means a measured value of the rotation velocity.

The velocity controller 55 includes a velocity instruction device 101, a deviation calculator 103, a PID controller 105, a static friction compensation device 107, and an adder 109. The velocity instruction device 101 outputs a target rotation velocity (or at each point of time from the start of control in accordance with the velocity profile input from the main controller 51.

The deviation calculator 103 calculates a deviation E_(V)=(ω_(r)−ω) between the target rotation velocity ω_(r) output from the velocity instruction device 101 and an actual rotation velocity w input from the measurement circuit 77. The PID controller 105 calculates a control input Uv for the conveyance motor 71 based on the deviation E_(V) input from the deviation calculator 103.

The PID controller 105 includes: a proportional element that amplifies the deviation E_(V) with a gain G_(p) and outputs it; an integral element that amplifies an integral value INT(E_(V)) of the deviation E_(V) with a gain G and outputs it; and a differential element that amplifies an integral value DIF(E_(V)) of the deviation E_(V) with a gain G_(d) and outputs it. The PID controller 105 calculates a total of the output from the proportional element, the integral element, and the differential element as the control input Uv for the conveyance motor 71.

The static friction compensation device 107 outputs a compensation amount C for compensating for the shortage of control input Uv caused by static friction. The compensation amount C is a fixed value under a condition that the actual rotation velocity w is zero, that is, in a static state. The compensation amount C is zero under a condition that the actual rotation velocity w is not zero, that is, in a non-static state.

The adder 109 corrects the control input Uv output from the PID controller 105 by the compensation amount C, and inputs, to the motor driver 73, a control input UPF=Uv C after correction. The motor driver 73 inputs, to the conveyance motor 71, a drive current corresponding to the control input UPF input from the velocity controller 55, and drives the conveyance motor 71 so that rotation torque corresponding to the control input UPF is generated. The rotation velocity of the conveyance roller 20 and the conveyance velocity of the sheet Q corresponding to the rotation velocity of the conveyance roller 20 are subjected to the feedback control by the velocity controller 55.

The tension control is executed by the tension controller 57 depicted in FIG. 6. The tension controller 57 calculates the control input U_(SR) for the supply motor 61 based on a deviation between tension of the sheet Q estimated from the position X of the tensioner 15 detected by the position detector 80 (hereinafter referred to as estimated tension) and the target tension. The tension controller 57 thus executes the feedback control for the tension of the sheet Q based on the control input U_(SR) calculated.

As depicted in FIG. 6, the tension controller 57 includes a tension instruction device 110, a tension estimator 120, a deviation calculator 130, a PID controller 140, a roll diameter estimator 150, a feedforward controller 160, an adder 170, and a gain setter (gain setting device) 180.

The tension instruction device 110 outputs target tension T_(r) at each point of time from the start of control in accordance with the tension profile input from the main controller 51. The tension estimator 120 estimates tension T acting on the sheet Q based on the position X of the tensioner 15 input from the position detector 80. Specifically, the tension estimator 120 can calculate, as estimated tension T, a value k·X obtained by multiplying the position X of the tensioner 15 by a certain proportional efficient k.

The deviation calculator 130 calculates a deviation E_(T)=T_(r)−T between the target tension T_(r) and the estimated tension T. The PID controller 140 calculates a feed back control input U_(B) for the supply motor 61 based on the deviation E_(T) input from the deviation calculator 130.

As depicted in FIG. 6, the PID controller 140 includes a proportional gain amplifier 141, an integral gain amplifier 142, a differential gain amplifier 143, an integrator 145, a differentiator 146, and an adder 148. The deviation E_(T) calculated by the deviation calculator 130 is input to the proportional gain amplifier 141, the integrator 145, and the differentiator 146. The proportional gain amplifier 141 amplifies the deviation E_(T) input from the deviation calculator 130 with the gain K_(p) and outputs it.

The integrator 145 executes integral calculation for the deviation E_(T) and inputs an integral value INT(E_(T)) of the deviation E_(T) to the integral gain amplifier 142. The integral gain amplifier 142 amplifies the integral value INT(E_(T)) of the deviation E_(T) input from the integrator 145 with a gain K_(i) and outputs it.

The differentiator 146 executes differential calculation for the deviation E_(T), and inputs a differential value DIF(E_(T)) of the deviation E_(T) to the differential gain amplifier 143. The differential gain amplifier 143 amplifies the differential value DIF(E_(T)) of the deviation E_(T) input from the differentiator 146 with a gain K_(d) and outputs it.

The adder 148 adds K_(p)·E_(T) output from the proportional gain amplifier 141, K_(i)·INT(E_(T)) output from the integral gain amplifier 142, and K_(d)·DIF(E_(T)) output from the differential gain amplifier 143. The adder 148 outputs an addition value K_(p)·E_(T) Ki·INT(E_(T))+K_(d)·DIF(E_(T)) as the feedback control input U_(B) for the supply motor 61.

The adder 170 outputs an addition value U_(B)+U_(F) obtained by adding the feedback control input U_(B) input from the PID controller 140 and a feedforward control input U_(F) input from the feedforward controller 160 as the control input U_(SR) for the supply motor 61.

The feedforward controller 160 includes a differentiator 161, an acceleration torque estimator 163, and an FF gain amplifier 165. The differentiator 161 differentiates the rotation velocity ω of the conveyance roller 20 input from the measurement circuit 77 to calculate rotation acceleration α of the conveyance roller 20. The rotation acceleration corresponds to angle acceleration. In the following, the rotation acceleration α calculated from the differentiator 161 is expressed as an actual rotation acceleration α.

The acceleration torque estimator 163 estimates acceleration torque τ of the supply motor 61 required for acceleration of the conveyance roller 20 based on the actual rotation acceleration α, in other words, required for rotation of the sheet roll Q0 depending on the acceleration of the sheet Q. Specifically, the acceleration torque τ is calculated in accordance with an equation τ=J(R)·(R_(P)/R)·α based on the actual rotation acceleration α, the roll diameter R that is a radius of the sheet roll Q0, a radius R_(P) of the conveyance roller 20, and an inertia J(R) of the sheet Q0 estimated from the roll diameter R.

Under a condition that the rotation acceleration of the conveyance roller 20 is a, the acceleration of the sheet Q conveyed by the rotation of the conveyance roller 20 is R_(P)·α. The sheet roll Q0 is required to rotate at rotation acceleration (R_(P)/R)·α to pull out the sheet Q from the sheet roll Q0 at the same acceleration. Under a condition that the inertia is J, the acceleration torque required for achieving this rotation is J·(R_(P)/R)·α.

A function J(R) for calculating the inertia J(R) of the sheet roll Q0 with the roll diameter being R is prepared in advance. The radius R_(P) of the conveyance roller 20 is a fixed value of the image forming system 1.

The roll diameter R of the sheet roll Q0 is estimated by the roll diameter estimator 150 based on the measurement signal output from the distance sensor 93. The measurement signal output from the distance sensor 93 indicates a distance Z between the surface of the sheet roll Q0 and the distance sensor 93. The distance from the center of the sheet roll Q0 to the distance sensor 93 is a fixed value Z0. The roll diameter estimator 150 can estimate the roll diameter R of the sheet roll Q0 by subtracting the distance Z from the fixed value Z0 (R=Z0−Z).

The acceleration torque estimator 163 calculates the acceleration torque τ based on the above equation and information of the roll diameter R input from the roll diameter estimator 150. The FF gain amplifier 165 adjusts the acceleration torque τ calculated so that the acceleration torque τ calculated is amplified by a gain K_(FF), and outputs acceleration torque K_(FF)·τ after adjustment as the feedforward control input U_(F). The gain K_(FF) is normally a value 1, and the gain K_(FF) may be finely adjusted from the value 1 depending on machine characteristics of a rotation system.

The motor driver 63 inputs a drive current corresponding to the control input U_(SR)=U_(F) U_(B) to the supply motor 61, and drives the supply motor 61 so that rotation torque corresponding to the control input U_(SR) is generated. The tension of the sheet Q is controlled to the target tension by executing the feedforward control and the feedback control for the supply motor 61.

In this embodiment, the gain setter 180 is configured to adjust the gains K_(p), Kc, and K_(d) in the PID controller 140 based on the roll diameter R estimated by the roll diameter estimator 150. The gains K_(p), K_(i), and K_(d) are set to K_(p)=K_(p)(R), =K_(i)(R), K_(d)=K_(d)(R) in accordance with the functions K_(p)(R), K_(i)(R), and K_(d)(R) of which variables are the roll diameter R. The functions K_(p)(R), K_(i)(R), and K_(d)(R) are determined in advance through an examination.

Specifically, the gain setter 180 adjusts the gains K_(p), K_(i), and K_(d) by repeatedly executing the processes in FIG. 7. Under a condition that the processes in FIG. 7 are started, the gain setter 180 calculates gains K_(p)=K_(p)(R), =K_(i)(R), K_(d)=K_(d)(R) to be set in the PID setter 140 based on the latest roll diameter R estimated by the roll dimeter estimator 150 (S210).

Further, the gain setter 180 determines whether the velocity profile is the constant velocity section (S220). Under a condition that the gain setter 180 has determined that the velocity profile is the constant velocity section, the gain setter 180 updates the gains K_(p), K_(i), K_(d) to values depending on the roll diameter R by setting the gains K_(p)=K_(p)(R), K_(i)=K_(i)(R), K_(d)=K_(d)(R) calculated in S210 in the PID setter 140 (S240). Then, the gain setter 180 ends the processes in FIG. 7.

Under a condition that the gain setter 180 has determined in S220 that the velocity profile is not the constant velocity section, the gain setter 180 corrects the gains K_(p)=K_(p)(R), =K_(i)(R), K_(d)=K_(d)(R) calculated in S210 to small values (S230). For example, the gain setter 180 corrects the gains K_(p)=K_(p)(R), =K_(i)(R), K_(d)=K_(d)(R) calculated in S210 to values K_(p)=h·K_(p)(R), K_(i)=h·K_(i)(R), K_(d)=h·K_(d)(R) obtained by multiplying the gains K_(p)=K_(p)(R), =K_(i)(R), K_(d)=K_(d)(R) by a coefficient h less than one.

After that, the gain setter 180 sets the gains corrected in S230 in the PID controller 140 (S240). After executing the process of S240, the gain setter 180 ends the processes in FIG. 7. The gain setter 180 repeats such processes.

The image forming system 1 of this embodiment described above differentiates the rotation velocity w of the conveyance roller 20 measured by the rotary encoder 75 and the measurement circuit 77 to specify the rotation acceleration α of the conveyance roller 20. Based on the rotation acceleration α of the conveyance roller 20 specified, the image forming system 1 calculates the acceleration torque τ of the supply motor 61 required for rotating the sheet roll Q0 depending on the acceleration of the conveyance roller 20.

Based on the acceleration torque τ calculated, the controller 50 calculates the feedforward control input U_(F) for the supply motor 61. Further, the controller 50 estimates the tension of the sheet Q based on the position X of the tensioner 15. The controller 50 calculates the feedback control input U_(B) for the supply motor 61 based on the deviation between the target tension T_(r) and the estimated tension T.

The controller 50 calculates the control input U_(SR) for the supply motor 61 based on the feedback control input U_(B) and the feedforward control input U_(F). Then, the controller 50 controls the supply motor 61 so that the drive current corresponding to the calculated control input U_(SR) is input to the supply motor 61.

In the image forming system 1, a component of the feedforward control input U_(F) included in the control input U_(SR) functions significantly during a process in which the sheet Q is conveyed with acceleration by rotation of the conveyance roller 20.

That is, the sheet roller Q0 rotates so that the sheet Q is pulled out from the sheet roll Q0 depending on the acceleration of the sheet Q by rotation of the conveyance roller 20. The acceleration torque depending on the inertia of the sheet roll Q0 is compensated by the feedforward control input U_(F).

In this embodiment, the gains K_(p), and K_(d) of the PID controller 140 at the time of acceleration are adjusted to be smaller than those at the time of the constant velocity so that the feedforward control functions significantly. This reduces the feedback control input U_(B). The rotation acceleration α is substantially zero in the constant velocity section. The feedforward control thus hardly functions, and the feedback control functions significantly.

In this embodiment, the sheet Q forming the sheet roll Q0 is reduced by use, which changes the radius R of the sheet roll Q0, the weight of the sheet roll Q0, and the inertia J(R). The controller 50 estimates the inertia J(R) of the sheet roll Q0 based on the radius R of the sheet roll Q0 estimated. Based on the inertia J(R) and the rotation acceleration α of the conveyance roller 20, the controller 50 estimates the acceleration torque τ depending on the inertia J(R).

According to this embodiment, the deviation E_(T) between the target tension T_(r) and the estimated tension T, the integral value of the deviation E_(T), and the differential value of the deviation E_(T) are amplified by amounts corresponding to the gains K_(p), K_(i), and K_(d), and the feedback control input U_(B) corresponding to the amplified values is calculated. The gains K_(p), and K_(d) are adjusted to values corresponding to the diameter R of the sheet roll Q0 as described above.

Thus, in this embodiment, the supply motor 61 is controlled appropriately immediately after a new sheet roll Q0 is installed in the holder 10, immediately before the sheet Q in the sheet roll Q finishes up, at the time of acceleration, and at the time of the constant velocity.

That is, the controller 50 inhibits the effect from a remaining amount and a motional state of the sheet roll Q0, and is capable of appropriately executing the conveyance velocity control and the tension control of the sheet Q so that they are linked with each other. Further, the controller 50 controls the conveyance velocity and the tension of the sheet Q with high accuracy to convey the sheet Q appropriately. Thus, it is possible to inhibit a skew of the sheet Q and errors in the conveyance velocity and the stop position of the sheet Q due to an excess or shortage of tension.

According to the technique of the present disclosure, it is possible to appropriately execute the conveyance control of the sheet Q with an acceleration process under the environment where the weight of the sheet roll Q0 changes depending on the consumption of the sheet Q.

In the above embodiment, the control input U_(SR) including the feedforward control input U_(F) and the feedback control input U_(B) is calculated for the supply motor 61 irrespective of whether the section is the acceleration section. However, the tension controller 57 may calculate the control input U_(SR) only including the feedback control input U_(B) in the constant velocity section. That is, the tension controller 57 may calculate the control input U_(SR) not to include the feedforward control input U_(F).

The tension controller 57 may calculate the control input U_(SR) only including the feedforward control input U_(F) in a non-constant velocity section, especially in the acceleration section. That is, the tension controller 57 may calculate the control input U_(SR) not to include the feedback control input U_(B).

The controller 50 can control the supply motor 61 based on at least one of the feedback control input U_(B) and the feedforward control input U_(F). During the acceleration of the conveyance roller 20, the controller 50 can control the supply motor 61 at least based on the feedforward control input U_(F) from among the feedback control input U_(B) and the feedforward control input U_(F). The controller 50 can control the supply motor 61 at least based on the feedback control input U_(B) during the rotation of the conveyance roller 20 at the constant velocity.

The holding structure of the sheet roll Q0 by the holder 10 and the driving system of the sheet roll Q0 are not limited to the above embodiment. In the image forming system 1 described above, the core material of the sheet roll Q0 is inserted into the rotation shaft 10A of the holder 10.

However, the holder may be formed from a hollow cylindrical material of which inside has an accommodation space for the sheet roll Q0. The holder may be configured so that an inner surface defining the accommodation space for the sheet roll Q0 rotates. The sheet roll Q0 may rotate depending on the rotation of the inner surface of the holder in a state of being accommodated in the holder. Further, a roller brought into contact with an outer circumferential surface of the sheet roll Q0 may be provided. The sheet roll Q0 may rotate by rotation of this roller.

Second Embodiment

Subsequently, the image forming system 1 of the second embodiment is explained. The image forming system 1 of the second embodiment is an image forming system partially different from the first embodiment. In the image forming system 1 of the second embodiment, the constitutive parts or components, which are the same as or equivalent to those of the first embodiment, are designated by the same reference numerals, and any explanation thereof is omitted as appropriate. Configurations of the image forming system 1 of the second embodiment that are different from the first embodiment are explained selectively.

In the second embodiment, the controller 50 includes a tension controller 200 depicted in FIG. 8 instead of the tension controller 57 depicted in FIG. 6. The tension controller 200 includes the tension instruction device 110, the tension estimator 120, the deviation calculator 130, the PID controller 140, the roll diameter estimator 150, the gain setter (gain setting device) 180, a primary delay filter 210, a target velocity generator 220, a deviation calculator 230, an adder 240, a supply velocity controller 250, a feedforward controller 260, and an adder 270.

The deviation calculator 130 calculates a deviation E_(T)=T_(r)−T between the target tension T_(r) output from the tension instruction device 110 and the estimated tension T output from the tension estimator 120 similar to the first embodiment. The PID controller 140 calculates a tension control input U_(T)=K_(p)·E_(T)+K_(i)·INT(E_(T))+K_(d)·DIF(E_(T)) based on the deviation E_(T) input from the deviation calculator 130.

The tension control input U_(T) corresponds to the feedback control input U_(B) of the first embodiment. The gain setter 180 is configured to adjust the gains K_(p), K_(i), and K_(d) in the PID controller 140 based on the roll diameter R estimated by the roll diameter estimator 150.

The target velocity generator 220 calculates a target rotation velocity ω_(sr) of the sheet roll Q0 based on the target rotation velocity ω_(r) of the conveyance roller 20 input from the velocity instruction device 101 of the velocity controller 55 via the primary delay filter 210 and the roll diameter R estimated by the roll diameter estimator 150.

The target velocity generator 220 calculates the target rotation velocity ω_(sr) in accordance with an equation ω_(sr)=(R_(P)/R)·ω_(r) so that the conveyance roller 20 and the sheet roll Q0 rotate at the same circumferential velocity. R_(p) is a radius of the conveyance roller 20 as described above. The target rotation velocity ω_(sr) of the sheet roll Q0 corresponds to the target rotation velocity ω_(sr) of the rotation shaft 10A.

The deviation calculator 230 calculates a deviation E_(W)=(ω_(sr)−ω_(s)) between the target rotation velocity ω_(sr) output from the target velocity generator 220 and the rotation velocity ω_(s) of the sheet roll Q0 measured by the measurement circuit 67. The rotation velocity ω_(s) of the sheet roll Q0 corresponds to an angular velocity of the sheet roll Q0 or the rotation shaft 10A.

The adder 240 calculates a control input U_(C)=(U_(T)+E_(W)) by adding the deviation E_(W) to the tension control input U_(T) output from the PID controller 140. The supply velocity controller 250 is configured as the PID controller to calculate a feedback control input U_(B)* by adding a velocity control component to the control input U_(C) output from the adder 240. Instead of the feedback control input U_(B) in the first embodiment, the feedback control input U_(B)* is input to the adder 270 in this embodiment.

The supply velocity controller 250 includes a proportional gain amplifier 251, an integral gain amplifier 252, a differential gain amplifier 253, an integrator 255, a differentiator 256, and an adder 258. The control input U_(C) output from the adder 240 is input to the proportional gain amplifier 251, the integrator 255, and the differentiator 256. The proportional gain amplifier 251 amplifies the control input U_(C) input with a gain K_(wp) and outputs it.

The integrator 255 inputs an integral value INT(U_(C)) of the control input U_(C) to the integral gain amplifier 252. The integral gain amplifier 252 amplifies the integral value INT(U_(C)) input with a gain K_(wi) and outputs it. The differentiator 256 inputs a differential value DIF(U_(C)) of the control input U_(C) to the differential gain amplifier 253. The differential gain amplifier 253 amplifies the differential value DIF(U_(C)) input with a gain K_(wd) and outputs it.

The adder 258 adds K_(wp)·U_(C) output from the proportional gain amplifier 251, K_(wi)·INT(U_(C)) output from the integral gain amplifier 252, and K_(wd)·DIF(U_(C)) output from the differential gain amplifier 253. Then, the adder 258 outputs an addition value K_(wp)·U_(C)+K_(wi)·INT(U_(c))+K_(wd)·DIF(U_(c)) as the feedback control input U_(B)*.

As depicted in FIG. 9, the feedforward controller 260 includes a differentiator 261, an acceleration torque estimator 263, a viscous friction estimator 265, a dynamic friction estimator 267, and adders 268, 269. The differentiator 261 differentiates the target rotation velocity ω_(r) of the conveyance roller 20 input from the primary delay filter 210 to calculate target rotation acceleration α_(r) of the conveyance roller 20.

The acceleration torque estimator 263 estimates, based on the target rotation acceleration α_(r), the acceleration torque τ of the supply motor 61 required for rotating the sheet roll Q0 depending on the acceleration of the conveyance roller 20. Specifically, the acceleration torque τ is calculated in accordance with an equation τ=J(R)·(R_(P)/R)·α_(r) based on the target rotation acceleration α_(r), the roll diameter R estimated by the roll diameter estimator 150, the radius R_(P) of the conveyance roller 20, and the inertia J(R) of the sheet Q0 estimated from the roll diameter R.

The viscous friction estimator 265 estimates viscous friction torque τ_(vf) in a rotary coordinate system of the sheet roll Q0 based on the target rotation velocity ω_(r) of the conveyance roller 20 input from the primary delay filter 210. The viscous friction torque τ_(vf) may be calculated in accordance with an equation τ_(vf)=C_(vf)·(R_(p)/R)·ω_(r). C_(vf) corresponds to a viscous friction coefficient. (R_(p)/R)·ω_(r) is the target rotation velocity ω_(sr) of the sheet roll Q0 or the rotation shaft 10A.

The dynamic friction estimator 267 estimates dynamic friction torque τ_(df) in the rotary coordinate system of the sheet roll Q0 based on the target rotation velocity for of the conveyance roller 20 input from the primary delay filter 210. Specifically, under a condition that the target rotation velocity for is not zero, the dynamic friction estimator 267 calculates, based on a dynamic friction coefficient C_(df), dynamic friction torque τ_(df)=C_(dr)·N(R) in the rotary coordinate system of the sheet roll Q0. N(R) is drag N depending on the roll diameter R.

Under a condition that the target rotation velocity for is zero, the dynamic friction estimator 267 calculates dynamic friction torque τ_(df)=0. Or, under the condition that the target rotation velocity for is zero, the dynamic friction estimator 267 may calculate, based on a static friction coefficient C_(sf), static friction torque τ_(sf)=C_(sf)·N(R) in the rotary coordinate system of the sheet roll Q0 as the dynamic friction torque τ_(df).

The adder 268 calculates friction torque τ_(f)=τ_(vf)+τ_(df) by adding the dynamic friction torque τ_(df) input from the dynamic friction estimator 267 and the viscous friction torque τ_(vf) input from the viscous friction estimator 265.

The adder 269 adds the friction torque τ_(f) input from the adder 268 to the acceleration torque τ input from the acceleration torque estimator 263 to calculate feedforward control input U_(F)*=τ_(f)+τ_(f). The adder 269 inputs the feedforward control input U_(F)* calculated to the adder 270. The feedforward control input U_(F)* corresponds to a control input obtained by adding a friction compensating component to the feedforward control input U_(F) calculated from the feedforward controller 160 of the first embodiment.

The adder 270 outputs an addition value U_(B)*+U_(F)* obtained by adding the feedback control input U_(B)* input from the supply velocity controller 250 and the feedforward control input U_(F)* input from the feedforward controller 260, as the control input U_(SR) for the supply motor 61.

According to the tension controller 200 of the second embodiment described above, it is possible to control the rotation of the sheet roll Q0 with high accuracy while including the friction torque caused by the rotary coordinate system of the sheet roll Q0.

The feedforward controller 260 of the second embodiment is different from that of the first embodiment and beneficial in that the feedforward control input U_(F)* is calculated not based on the actual rotation velocity w of the conveyance roller 20 but based on the target rotation velocity ω_(r).

A power transmission system such as a gear is provided between the supply motor 61 and the rotation shaft 10A of the holder 10. Thus, there is a time lag until driving of the supply motor 61 is reflected in the rotary motion of the sheet roll Q0. The time lag may cause a control error if the feedforward control input U_(F) is calculated based on the actual rotation velocity ω of the conveyance roller 20 to control the supply motor 61.

In the second embodiment, the feedforward control input U_(F)* is calculated based on the target rotation velocity ω_(r) of the conveyance roller 20. In this case, the rotary motion of the sheet roll Q0 can be controlled by controlling the supply motor 61 while inhibiting the effect of the time lug.

Under the condition that the feedforward control input U_(F)* is calculated based on the target rotation velocity ω_(r), tensioning for the sheet Q is preferably performed before the conveyance process of the sheet Q is started so that the target rotation velocity (Dr indicates movement or motion of the sheet Q well.

In the second embodiment, before the conveyance process of the sheet Q is started in S120, the main controller 51 executes a tensioning process (S115) indicated in FIG. 10. In S115, in a state of stopping the conveyance roller 20, the main controller 51 controls the tension controller 57 to rotate the supply motor 61 in a reverse direction until the estimated tension T reaches reference tension TO.

Under a condition that the supply motor 61 rotates in a normal direction, the sheet Q is conveyed or sent in the conveyance direction. In S115, rotating the supply motor 61 in the reverse direction in the state of stopping the conveyance roller 20 rewinds part of the sheet Q to the sheet roll Q0, and thus tension is applied to the sheet Q.

The reference tension TO is the target tension T_(r) under the condition that the conveyance process of the sheet Q is started in S120 or tension in the vicinity thereof. In the conveyance process (S120) of the sheet Q after the tensioning process (S115) is executed, the tension of the sheet Q at the beginning of the conveyance process is substantially the same as the target tension T_(r), and the conveyance roller 20 and the sheet roll Q0 rotate at substantially the same circumferential velocity.

In the conveyance process (S120) executed after the tensioning process (S115), the target rotation velocity (Dr indicates actual motion or movement of the sheet Q well and it is possible to appropriately execute the conveyance control of the sheet Q.

The exemplary embodiments of the present disclosure including the first embodiment and the second embedment are explained above. The present disclosure, however, is not limited to the exemplary embodiments described above and can adopt various aspects.

For example, the tensioning process (S115) may be executed in the first embodiment. Similar to the feedforward controller 260 in the second embodiment, the feedforward controller 160 in the first embodiment may estimate the viscous friction torque and/or the dynamic friction torque and may calculate the feedforward control input U_(F) by adding the viscous friction torque and/or the dynamic friction torque to the acceleration torque τ. In that case, the feedforward controller 160 may estimate the viscous friction torque and/or the dynamic friction torque not based on the target rotation velocity ω_(r) but based on the actual rotation velocity ω. Similarly, instead of the target rotation velocity ω_(r), the actual rotation velocity ω may be input to the feedforward controller 260 of the second embodiment.

The technique of the present disclosure may be applied to various image forming systems. The technique of the present disclosure may be applied to an image forming system not including the belt mechanism 30. In this case, the image forming system may include a platen for supporting the sheet Q, instead of the belt mechanism 30.

The technique of the present disclosure may be applied to an image forming system in which a recording head of a serial driving system is provided as the recording head 40 instead of the line head. In this case, the recording head forms an image on the sheet Q by reciprocatingly moving in the line direction. The technique of the present disclosure may be applied to an image forming system of an electrophotographic system.

The technique of the present disclosure may be applied to a system for forming an image on a surface of the sheet Q that faces the outside in a radial direction of the sheet roll Q0. Or, the technique of the present disclosure may be applied to a system for forming an image on a back surface of the sheet Q that faces the inside in the radial direction of the sheet roll Q0. The technique of the present disclosure may be applied to a system for forming an image on both surfaces of the sheet Q.

The technique of the present disclosure can be applied not only to the system for forming an image on the sheet Q by use of a color material but also to a variety of systems. For example, the technique of the present disclosure may be applied to a system for making a mark in the sheet Q through perforation or to a system for irradiating a surface of the sheet Q with light to sterilize the surface. The technique of the present disclosure may be applied to a system for forming a trace pattern on a sheet-like substrate. The sheet roll Q0 and the sheet Q may be paper, vinyl, or a flexible printed board (FPC).

The configuration(s) of the tensioner 15 and the tension estimator 120 is/are not limited to the above embodiments. The tensioner may be configured as an arm in which the first end is pivotally supported and the second end has a roller, like a pendulum arm. The tensioner may include an actuator to apply tension to the sheet Q, and tension may be estimated from a change amount of the actuator. A dedicated sensor may be provided to estimate tension. The sensor may act on the tensioner 15 or the sheet Q to detect tension. The configuration(s) of the rotary encoders 65 and 75 are not limited to the above embodiments. The rotary encoders 65 and 75 may not be optical rotary encoders, but magnetic rotary encoders.

In the above embodiment(s), the roll diameter R is measured by using the distance sensor 93. The roll diameter R may be estimated without using the distance sensor 93. For example, the roll diameter R may be estimated from the conveyance amount of the sheet Q of the conveyance roller 20 and the rotation amount of the sheet roll Q0 corresponding thereto. The conveyance amount of the sheet Q can be specified by the rotation amount of the conveyance roller 20. The rotation amounts (i.e., rotation angles) of the conveyance roller 20 and the sheet roll Q0 can be measured based on the outputs from the rotary encoders 65 and 75.

The printing controller 53, the velocity controller 55, and the tension controllers 57, 200 may be configured by combining the CPU and the ASIC. In each of the controller 50, the main controller 51, the printing controller 53, the velocity controller 55, and the tension controllers 57, 200, the number of the CPU(s) and the ASIC(s) and whether or not the CPU and/or the ASIC is/are provided therein is not limited to the above specific examples.

The PID controllers 105, 140 used for feedback control may be replaced by any other controller such as a PI controller. Part of the gains K_(p), K_(i), and K_(d) may not be updated based on the roll diameter R. In the second embodiment, the gains K_(wp), K_(wi), and K_(wd) may be updated based on the roll diameter R similarly to the gains K_(p), K_(i), and K_(d).

The function provided in one component in each of the above exemplary embodiments may be distributed in components. The function provided in components may be integrated in one component. Part of the configuration according to each of the above exemplary embodiments may be omitted. The embodiments of the present disclosure include various embodiments or aspects that are included in the technical ideas specified by the following claims.

There is a correspondence relationship between the words and terms as follows. The conveyance motor 71 corresponds to an exemplary first motor. The supply motor 61 corresponds to an exemplary second motor. The rotary encoder 75 and the measurement circuit 77 correspond to an exemplary measuring device. The distance sensor 93 and the roll diameter estimator 150 correspond to an exemplary roll diameter measuring device. The rotary encoder 65 and the measurement circuit 67 correspond to an exemplary roll measuring device. 

What is claimed is:
 1. A sheet conveyor, comprising: a holder configured to detachably hold a sheet roll; a conveyance roller configured to convey, by rotation, a sheet pulled out from the sheet roll; a first motor configured to rotate the conveyance roller; a second motor configured to rotate the sheet roll along with conveyance of the sheet; a tension estimator configured to estimate tension of the sheet conveyed by the rotation of the conveyance roller; a measuring device configured to measure a physical quantity related to rotary motion of the conveyance roller; and a controller, wherein the controller is configured to: estimate acceleration torque of the second motor required for rotating the sheet roll depending on acceleration of the conveyance roller, based on rotation acceleration of the conveyance roller specified by the physical quantity measured by the measuring device; calculate a feedforward control input for the second motor based on the acceleration torque estimated; calculate a feedback control input for the second motor based on target tension and estimated tension as the tension estimated by the tension estimator; control the second motor based on at least one of the feedback control input calculated and the feedforward control input calculated; and control the second motor at least based on the feedforward control input from among the feedback control input and the feedforward control input, under a condition that the conveyance roller is accelerated.
 2. The sheet conveyor according to claim 1, wherein the controller is further configured to: calculate a control input for the first motor based on a deviation between a target rotation velocity and a rotation velocity of the conveyance roller specified by the physical quantity measured by the measuring device; and control the first motor based on the control input calculated.
 3. A sheet conveyor, comprising: a holder configured to detachably hold a sheet roll; a conveyance roller configured to convey, by rotation, a sheet pulled out from the sheet roll; a first motor configured to rotate the conveyance roller; a second motor configured to rotate the sheet roll along with the conveyance of the sheet; a tension estimator configured to estimate tension of the sheet conveyed by the rotation of the conveyance roller; a measuring device configured to measure a physical quantity related to rotary motion of the conveyance roller; and a controller, wherein the controller is configured to: control the first motor based on the physical quantity measured by the measuring device, such that the conveyance roller rotates in accordance with a velocity profile, estimate acceleration torque of the second motor required for rotating the sheet roll depending on acceleration of the conveyance roller, based on target rotation acceleration of the conveyance roller specified by the velocity profile; calculate a feedforward control input for the second motor based on the acceleration torque estimated; calculate a feedback control input for the second motor based on target tension and estimated tension as the tension estimated by the tension estimator; control the second motor based on at least one of the feedback control input calculated and the feedforward control input calculated; and control the second motor at least based on the feedforward control input from among the feedback control input and the feedforward control input, under a condition that the conveyance roller is accelerated.
 4. The sheet conveyor according to claim 1, wherein the controller is configured to: determine a diameter of the sheet roll; and estimate the acceleration torque based on the diameter of the sheet roll.
 5. The sheet conveyor according to claim 1, wherein the controller is configured to: determine a diameter of the sheet roll; calculate inertia in a rotary coordinate system of the sheet roll based on the diameter of the sheet roll; and estimate the acceleration torque based on the inertia and rotation acceleration of the conveyance roller.
 6. The sheet conveyor according to claim 3, wherein the controller is configured to: determine a diameter of the sheet roll; calculate inertia in a rotary coordinate system of the sheet roll based on the diameter of the sheet roll; and estimate the acceleration torque based on the inertia and the target rotation acceleration of the conveyance roller.
 7. The sheet conveyor according to claim 5, wherein the controller is configured to: estimate viscous friction torque in the rotary coordinate system of the sheet roll based on the diameter of the sheet roll and the physical quantity measured by the measuring device; and calculate the feedforward control input for the second motor based on the acceleration torque and the viscous friction torque estimated.
 8. The sheet conveyor according to claim 6, wherein the controller is configured to: estimate viscous friction torque in the rotary coordinate system of the sheet roll based on the diameter of the sheet roll and the velocity profile; and calculate the feedforward control input for the second motor based on the acceleration torque and the viscous friction torque estimated.
 9. The sheet conveyor according to claim 7, wherein the controller is configured to: further estimate dynamic friction torque; and calculate the feedforward control input for the second motor based on the acceleration torque, the viscous friction torque, and the dynamic friction torque estimated.
 10. The sheet conveyor according to claim 4, further comprising a roll diameter measuring device configured to measure the diameter of the sheet roll, wherein the controller is configured to determine the diameter of the sheet roll measured by the roll diameter measuring device based on input from the roll diameter measuring device.
 11. The sheet conveyor according to claim 4, wherein the controller is configured to: calculate the feedback control input based on a value obtained by amplifying an input value by an amount corresponding to a gain, the input value being at least one of a deviation between the estimated tension and the target tension, an integral value of the deviation, and a differential value of the deviation; and set the gain to a value corresponding to the diameter of the sheet roll.
 12. The sheet conveyor according to claim 1, further comprising a roll measuring device configured to measure a physical quantity related to rotary motion of the sheet roll, wherein the controller is configured to calculate a control input as the feedback control input, based on a tension deviation and a velocity deviation, the tension deviation is a deviation between the estimated tension and the target tension, and the velocity deviation is a deviation between a target rotation velocity of the sheet roll and a rotation velocity of the sheet roll specified by the physical quantity measured by the roll measuring device.
 13. The sheet conveyor according to claim 3, further comprising a roll measuring device configured to measure a physical quantity related to rotary motion of the sheet roll, wherein the controller is configured to calculate a control input as the feedback control input, based on a tension deviation and a velocity deviation, the tension deviation is a deviation between the estimated tension and the target tension, and the velocity deviation is a deviation between a target rotation velocity of the sheet roll based on a target rotation velocity of the conveyance roller specified from the velocity profile and a rotation velocity of the sheet roll specified from the physical quantity measured by the roll measuring device.
 14. The sheet conveyor according to claim 1, wherein the controller is configured to: control the second motor based on both of the feedback control input and the feedforward control input, under the condition that the conveyance roller is accelerated; control the second motor at least based on the feedback control input, under a condition that the conveyance roller is rotated at a constant velocity; calculate the feedback control input based on a value obtained by amplifying an input value by an amount corresponding to a gain, the input value being at least one of a deviation between the estimated tension and the target tension, an integral value of the deviation, and a differential value of the deviation; and in a case of accelerating the conveyance roller, set the gain to a value lower than a case of rotating the conveyance roller at the constant velocity.
 15. The sheet conveyor according to claim 1, wherein the controller is configured to: determine an addition value obtained by adding the feedback control input calculated and the feedforward control input calculated, as the control input for the second motor, irrespective of whether the conveyance roller is accelerated; and control the second motor based on the control input determined.
 16. The sheet conveyor according to claim 1, further comprising a tensioner being movable and configured to apply tension to the sheet by contacting with the sheet pulled out from the sheet roll, wherein the tension estimator is configured to estimate the tension of the sheet based on a position of the tensioner that is moved by the contact with the sheet.
 17. The sheet conveyor according to claim 1, wherein the controller is configured to control the second motor such that the tension of the sheet corresponds to the target tension before driving of the conveyance roller is started by the first motor.
 18. An image forming system comprising: the sheet conveyor as defined in claim 1; and a recording unit configured to form an image on the sheet conveyed by the sheet conveyor. 